1. Field of the Invention
The present invention relates to a semiconductor device and a driving method thereof.
2. Description of the Related Art
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices have actively developed. In particular, a technique for forming a driver circuit such as a gate driver over the same substrate as a pixel portion with the use of a transistor formed using a non-single-crystal semiconductor has actively developed because the technique greatly contributes to reduction in manufacturing cost and improvement in reliability.
However, a transistor formed using a non-single-crystal semiconductor deteriorates. Accordingly, the decrease in mobility, the rise (or the fall) in the threshold voltage, or the like occurs. In particular, in a gate driver, a transistor having a function of applying negative voltage (also referred to as an L-level potential) to a gate signal line (such a transistor is also referred to as a pull-down transistor) greatly deteriorates. This is because the pull-down transistor is turned on so as to apply negative voltage to the gate signal line in the case where the gate signal line is not selected. In other words, the pull-down transistor is on in most of the one frame period because the gate signal line is not selected.
In order to solve the foregoing problems, Reference 1 discloses a gate driver where deterioration of a pull-down transistor can be suppressed. Reference 1 discloses a circuit capable of outputting pulses (e.g., a holding control portion 350 in FIG. 7 in Reference 1) that is provided in each stage of the gate driver in order to suppress deterioration of the pull-down transistor. The conduction state of the pull-down transistor is controlled with an output signal of the circuit. The circuit outputs a pulse in synchronization with a clock signal or the like. Therefore, the length of time during which the pull-down transistor is on can be decreased, so that deterioration of the pull-down transistor can be suppressed. However, the circuit capable of outputting pulses includes a transistor Q32 which is on in most of the one frame period. Therefore, the transistor Q32 deteriorates.